1. Field of the Invention
The present invention relates to a dielectric resonator device operating in a multiple mode, and also to a dielectric filter, a composite dielectric filter, and a communication apparatus provided with the above type of dielectric resonator device.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 11-145704 discloses a multiple-mode dielectric resonator device utilizing a plurality of resonant modes in which a dielectric core is disposed in a cavity.
In the dielectric resonator device of the above-described publication, a dielectric core generally formed in a rectangular parallelepiped is disposed at the center of a cavity generally formed in a rectangular parallelepiped. Support bases having a low dielectric constant are attached by glass glazing to the dielectric core, which is integrally formed with the cavity, provided with provisional support members, and are then fired. Subsequently, the provisional support members are removed. The structure in which a dielectric core is supported at the center of the cavity by disposing a support base under the dielectric core is also disclosed in this publication.
However, the structure in which the dielectric core is supported in the cavity by providing the provisional support members presents the following problems:
(1) complexity of the manufacturing process;
(2) necessity to increase the dimensional precision of the dielectric core, the cavity, and the support bases; and
(3) complexity of molding dies for the dielectric core and the cavity.
Because of the above-described problems, the cost of the resulting dielectric resonator device becomes high.
In the structure in which the dielectric core is disposed in the cavity by disposing the support base therebetween, the effective dielectric constant in the direction in which an electric field vertically passes through the contact face between the support base and the dielectric core is increased by the height of the support base, which serves as a dielectric member, and accordingly, the resonant frequency of the resonant mode (TM mode) in the above-described direction is decreased. If the TM mode is an unwanted resonant mode, this adversely influences the characteristics, for example, filter attenuation characteristics, obtained by the other resonant modes. Even if the TM mode is utilized, the dielectric resonator device must be designed by taking into consideration the decrease in the resonant frequency due to the presence of the support base, thereby decreasing designing flexibility.